The present invention is concerned with high performance vehicle tires, i.e., tires having high inflation pressures. While the following discussion is primarily concerned with tires for racing bicycles, the concepts are applicable to tires for other vehicles where similar performance characteristics are desired.
Racing bicycle tires are commonly called "sew ups". They look somewhat like an ordinary bicycle inner tube with tread, but they are constructed quite differently. They comprise a casing in the form of a toroid enclosing a thin lightweight inner tube made of either butyl rubber or latex rubber. The latex tubes are lighter, but they are also somewhat air permeable, so much so that they loose significant pressure after a few hours.
These tires are constructed to minimize their weight as well as that of the wheel on which they are mounted. They are also designed to operate at high inflation pressures (in excess of seventy-five pounds per square inch, or 5.273 Kg./sq. cm.). The lightweight minimizes the moment of inertia of the tire and wheel, thus keeping the work required to accelerate the bicycle to a minimum. The high inflation pressures help reduce rolling resistance.
The casing of the commercially available racing tires is a rubber casing reinforced with bias ply unidirectional roving (individual threads) made of either cotton or silk. This casing is made as light as possible and it contains a high percentage of fiber and a low percentage of rubber in order to minimize the weight. However, due to its lightweight construction it is fragile. A sharp blow caused by hitting a stone or chuck hole causes a weakening of the rubber bond between single fibers of the unidirectional roving, and the roving separates at the weakened point. If this does not result in instant failure, it permits a blister to form, a blister which is pushed outward by the high inflation pressure in the inner tube. Such a blister is an incipient point of rapid failure.
The casing is sewn together at the inside with either cotton or linen thread. This seam is used during construction for inserting the inner tube and it also permits access to the tube for repairs in the event of a puncture caused by a sharp object such as a nail. It is impossible to repair the assembly if there is serious damage to the casing.
In the present tires a rubber tread piece is bonded to the casing with rubber. This tread piece serves as a point of wear; it provides added protection to the casing along the line of road contact; and it provides a piece in which a tread pattern can be formed. Tread patterns are varied depending upon the intended running surface and condition (i.e., concrete road or wooden track, wet or dry surface, etc.).
These present tires weigh a minimum of approximately ninety grams to a maximum of approximately four hundred grams. A very light tire intended for track racing might weigh ninety grams (although a one hundred and ninety gram tire is considered to be excellent) while a heavy tire intended for use on rough roads might weigh four hundred grams. These weights are that of the complete tire which has a tube enclosed therein.
These present tires present significant problems for the user. They are easily damaged and thus they have very short life expectancy. The construction using unidirectional roving bonded together with rubber puts a premium on the rubber bond and the quality of this bond is difficult to maintain under production conditions. Premature failure of casings very early in their life is not common, even after only a few miles of riding.
One possible improvement would be to substitute a strong synthetic material (nylon, polyester, glass, or kevlar) for the cotton or silk unidirectional bias ply roving. This has probably not been done in the present tires because of the difficulty in obtaining an adequate bond between the rubber matrix and these synthetics. There is very little rubber available (if the weight of the tire is to be held down) and rubber will not bond as well to the smooth long fiber synthetics as it will to the natural cotton or silk. As an added argument against this substitution, the present failures are seldom caused by fracture of the fibers in the casing, but the failures are more often caused by tearing the rubber bonding material which holds the fibers together.
My invention comprises replacing the troublesome casing made of unidirectional roving and rubber with a casing made of a synthetic fiber woven cloth. Using woven cloth assures a strong assembly in all directions, eliminating the weakness of the rubber bond. Any strong synthetic fiber such as nylon, polyester, glass, or kevlar might be used, but I have been using a Dacron polyester in my prototypes. Desirably the cloth would not have a surface treatment which would tend to weaken its fibers. The smaller the memory error in the plastic of which the cloth threads are formed the better will be the resulting tire as hereinafter discussed. An advantage in the use of plastic (synthetic material) threads in the cloth resides in the fact that such threads will have a hard, smooth surface as compared to threads of a natural material. Thus, the threads are better able to slide on each other and the resulting tire will have less memory and be a better riding tire. To this cloth casing I bond a polyurethane tread piece. Polyurethane has very high abrasion and tear resistance as compared with rubber or other plastics. In the right formulations and durometers it has a high coefficient of static friction and a low coefficient of rolling friction. Other synthetic materials such as nitril rubber or natural rubber could be similarly employed.
J. B. Dunlop is usually regarded as the inventor of the pneumatic tire. His initial construction was a toroid consisting of a layer of rubber impregnated, non-synthetic fabric alone or sandwiched between two layers of rubber. See the book "The History Of The Pneumatic Tyre" by J. B. Dunlop, A. Thoms & Co. Ltd. [probably 1925]. This form of tire lasted only briefly before being replaced by the "clincher" type design having an omega cross-section. See the book "Encyclopedia Of Polymer Science & Technology", John Wiley and Sons (1971), Vol. 14, pg, 42. Thus it was, perhaps, nothing more than an abandoned experiment. In any event, such an impregnated fabric would be unsuitable for the high performance tire with which the present invention is concerned. In the first place such impregnation adds weight, a very undesirable characteristic. Secondly, it prevents the fibers or threads of the cloth from moving with respect to each other when the tire is in use. I believe that such movement contributes to the desirable characteristics of my tire.
A tire made in accordance with my invention can easily be within the "excellent" weight range for tires of conventional construction. At the same time the other advantages discussed herein are obtained.
The rims on which racing bicycle tires are used do not have flanges between which the tire is received, as is the case with the usual pleasure bicycle. Instead, the periphery of the rim is only slightly concave (in transverse cross-section) to form a saddle in which the tire is seated. It is the practice to hold the tire in this saddle by means of an adhesive, but if the bond fails the tire is likely to come off the rim.
I have found that by cutting the cloth body for my tire casings so that the woof and warp threads run in a bias direction (i.e., about forty-five degrees to the elongated dimension of the blank from which the casing is formed), that the tires tighten down nicely onto the rims when they are inflated. By making them this way, the material stretches in a bias fashion and the circumference of the tire shortens as the inflation pressure is increased. This also assures that the outer fibers of the tire are highly tensioned when the tire is used, so any wave in the tire surface ahead of the point of contact with the road will be minimized. The larger the wave, the greater will be the rolling resistance. It also has the advantage of compensating for minor size variations in the circumferential size of the casing so that manufacturing tolerances need not be extremely critical. Correspondingly, the gluing of the tires to the rims does not become as critical. In the Dunlop book, supra, the bias alignment of the cloth is mentioned, but indicated to be undesirable. In contrast, I obtain important advantages therefrom.
My present casings are cut from flat pieces of hard woven cloth and seamed at the ends to form a hoop. It is important to cut the end seams on a bias and sew them on a bias to match the thread lines of the woven cloth. When the seaming does not match the cloth thread lines, the cloth stretches or pulls unevenly at the end seams and creates bumps or hollows. The end seams may be cut with a single bias cut, or with multiple cuts to form a "V" or a "W" on the end seam. It would also be possible to weave the cloth casing in the form of a hoop, eliminating the end seam entirely.
Other objects and advantages of the present invention will be apparent from the following description.